1. Field of the Invention
The present invention relates to a method for manufacturing cold-rolled steel sheet.
2. Description of the Related Arts
Cold-rolled steel sheets are widely used as basic materials for exterior sheets of automobiles and other equipment. Since the major form of the cold-rolled steel sheets for automobiles is press-formed members, various kinds of workability characteristics are required responding to the shapes of the members. In particular, automobile-use requests the cold-rolled steel sheets for press-forming having excellent deep-drawing performance suitable for exterior sheets for automobiles. Recently, the request of automobile manufacturers relating to rationalization becomes severer than ever, particularly in the request for cost reduction of base materials and for improvement in production yield. To cope with these requirements, the material manufacturing faces serious issues of rationalization of manufacturing method, improvement of material quality, and homogeneity of material.
Based on the above-described background, and in view of rationalization of manufacturing method and improvement of material quality, JP-B-60-45692, (the term xe2x80x9cJP-B-xe2x80x9d referred to herein signifies the xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d), discloses a technology for improving the surface properties and the deep-drawing performance of a steel sheet using a process of continuous casting and direct feeding to rolling by hot-rolling a very low carbon steel slab containing not more than 0.015% C, wherein the hot rolling is begun in a range of temperature of the surface at center of the slab width from 600xc2x0 C. to less than 900xc2x0 C., and applying soaking within a period of 30 minutes during the hot-rolling step.
From the point of improvement of material quality, JP-A-5-112831, (the term xe2x80x9cJP-A-xe2x80x9d referred to herein signifies the xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), discloses a technology for improving the r value by applying a final reduction in thickness during the hot-rolling to 30% or more, and by beginning rapid cooling immediately after the completion of hot-rolling, thus reducing the grain size in the hot-rolled steel sheet.
The above-described prior arts, however, leave a problem on the uniformity of mechanical properties within a coil, though the surface properties and the deep drawing performance of the cold-rolled steel sheet are improved to a relatively favorable level. That is, the technology of JP-B-60-45692 adopts the heating temperature in the hot-rolling step to a low level, or to the ferritic domain. Accordingly, the congregation texture of the steel sheet after the hot-rolling differs in the width direction thereof owing to the temperature distribution in the material width direction during the rolling, (temperature reduction is significant at edges and peripheral zone thereof). As a result, the mechanical properties of the steel sheet in the coil width direction induce dispersion after cold-rolling and annealing.
If the structure and the mechanical properties of the steel sheet in the coil width direction generated dispersion, the workability within a plane of the material becomes non-homogeneous. Particularly when superior deep drawing performance is requested for the exterior sheets of automobiles and other uses, the quality of press-formed steel sheets have variations (such as cracks and wrinkles). Consequently, the automobile manufacturers have to apply blank layout in a coil under a low yield condition, (or to apply blank layout in a non-reasonable direction such as 45 degrees, or the product is not cut from nearby zone to coil edges).
Also in the technology of JP-A-5-112831, the dispersion of material quality can not necessarily be reduced to a satisfactory level. That is, with the range of cooling speed that is a feature of the technology, (according to the examples given in JP-A-5-112831, the average cooling speed in a period of one second from the start of cooling ranges from 90 to 105xc2x0 C./sec, and the average cooling speed in a period of 3 seconds after the start of cooling ranges from 65 to 80xc2x0 C./sec), the time until the start of cooling becomes long under the commercial hot-rolling conditions because particularly the cooling speed at top section of the rolling is slow, which allows the enhancement of coarse grain formation owing to the austenitic grain growth. Consequently, it was found that these sections are not necessarily able to prepare fine grains in the hot-rolled steel sheet.
In addition, the cooling immediately after the hot-rolling, which is a feature of the technology, is difficult to be actualized on commercial facilities because of the structural limitation thereof. That is, instruments have to be installed so that the cooling unit cannot be positioned directly next to the exit of the final stand of the finish rolling mill. Therefore, to bring the time to start cooling after completed the hot-rolling to 0.1 second or less is substantially difficult. Furthermore, since the technology adopts a large reduction in thickness, 30% or more, at the final stand of the finish rolling mill, the travel of steel sheet becomes unsteady and likely induces bad sheet shapes. With the bad shapes of hot-rolled coil sheet, users have a problem of unable to perform press-forming at a high yield.
As described above, practical application of the technology of JP-A-5-112831 has many issues yet to be solved.
In this regard, an object of the present invention is to provide a method for manufacturing cold-rolled steel sheet for deep drawing, which method solves the above-described problems of prior art, and allows to manufacture cold-rolled steel sheets suitable for the uses as exterior sheets for automobiles and other uses, giving superior press-formability with less variations in press-formability within a coil, on an industrially stable basis.
Another object of the present invention is to provide a method for manufacturing cold-rolled steel sheet for deep drawing, which method allows to manufacture cold-rolled steel sheets having superior sheet shape adding to the advantages described above, on an industrially stable basis.
As for the cold-rolled steel sheet and the surface-treated steel sheet, which are required to have good workability, they need to have mechanical properties of superior elongation and deep drawing performance, and less anisotropic property. The shape of steel sheet and the transferability of the hot-rolled steel strip during manufacturing process are also important variables to manufacture that kind of steel sheet.
According to prior art, mildness and high ductility are gained in very low carbon and nitrogen base compositions by adding elements to form carbide and elements to form nitride, such as Ti and Nb. The concept is based on that the interstitial elements such as carbon and nitrogen are eliminated as far as possible during the steel making stage, and that the interstitial elements at a level being left non-eliminated or the interstitial element at a level that cannot be eliminated on an economical basis are fixed as precipitates, thus rejecting the presence of interstitial elements in the steel.
With the increasing severity in requirements for workability, however, sole composition adjustment cannot anymore provide steel sheets that satisfy the requirements, and the manufacturing process is requested to contribute to further improvement of the material quality. It is known that, in concept, the effective use of the cooling technology improves the mechanical properties of steel sheets after cooling and annealing by reducing the grain size in the hot-rolled steel sheets. The procedure is to simultaneously apply the following-given two steps to reduce the grin size in the hot-rolled steel sheets: (1) to shorten the time between the completion of the hot-rolling and the start of the cooling step, (hereinafter referred to as the xe2x80x9ctime to start coolingxe2x80x9d), and (2) to increase the cooling speed as far as possible.
The basis of the technology is the following. For the step (1), since the strain which is induced during the finish-rolling recovers to induce recrystallization after completing the hot-rolling, as well as the xcex3 (austenite) grain growth promptly begins, (a) the cooling starts when the xcex3 grains are still in small size, and the xcex1 (ferrite) grains are formed from the fine xcex3 grain boundaries, thus generating fine grains, or (b) the cooling starts within further short time to form xcex1 grains as the deformation band in xcex3 grains as the nuclei in a state that the work strain during the hot-rolling step is not fully released, thus achieving the formation of fine grains.
As for the above-described step (2), when the cooling speed is slow, the recovery and recrystallization of xcex3 grains and grain growth occur during the cooling step, and the growth of xcex1 grains occurs after the transformation, thus the cooling speed is increased to achieve the reduction of xcex1 grain size. In addition, there is an advantage that, by increasing the cooling speed, the xcex3xe2x88x92xcex1 transformation point is lowered, and the grain growth after the transformation is suppressed to a magnitude corresponding to the reduced temperature after the transformation.
In view of experimental studies, for example, Zairyo To Process (Current Advance in Materials and Processes), Kino et al. vol.3, p.785 (1990) discloses a finding that, when the grain size reduction in a hot-rolled steel sheet is carried out by applying the finish temperature held to Ar3 transformation point or higher level, and applying (a) the cooling starting after 0.1 second from the completion of hot-rolling, then applying (b) the cooling with about 180xc2x0 C./sec of the cooling speed, then the mechanical properties, particularly the r value, after cold-rolled and annealed are improved.
Regarding the material quality improvement by applying cooling to reduce the grain size in hot-rolled steel sheet, various methods for manufacturing thereof have been disclosed. For example, JP-A-7-70650 discloses a method for achieving 2.50 or higher r value with a very low carbon (15 ppm or less C) steel sheet. According to the method, the finish-rolling is completed at Ar3 transformation point or higher temperature, then the time to start cooling is set to within 0.5 second after completing the rolling, and the cooling is conducted at cooling speeds of from 50 to 400xc2x0 C./sec over the temperature range of from the cooling start temperature to the (Ar3 transformation pointxe2x88x9260xc2x0 C.). The method, however, specifies the cumulative reduction in thickness in 3 passes at the exit side of the finish-rolling of hot-rolling to 50% or more. The method aims to actualize 2.50 or higher r value and deep drawing performance through the grain size reduction in the hot-rolled steel sheet using the cooling technology and through the accumulation of large quantity of work strain in the hot-rolling step.
With the technology disclosed by Kino et al. and the technology disclosed in the above-given patent publications, however, all the mechanical properties including r values cannot necessarily be always satisfied under all kinds of conditions. And, under some conditions, the workability such as the r value and the elongation are not improved, or rather degraded. On accumulating large amount of work strain during the hot-rolling step, the shape of steel sheet may be disturbed to induce problems on transferability of the steel sheet. That is, there has not been attained process condition that stably manufactures steel sheets having superior shape and transferability, and having significantly superior workability such as r value and elongation, in prior art.
The present invention was completed to cope with the above-described problems, and an object of the present invention is to provide a method for manufacturing cold-rolled steel sheet that has a very low carbon and nitrogen basis composition and that has the superior shape property including transferability, the superior workability, and the superior less-anisotropic property.
It is an object of the present invention as the first aspect thereof to provide a method for manufacturing cold-rolled steel sheet for deep drawing, which cold-rolled steel sheet is suitable for exterior sheets of automobiles and the like, has excellent press-formability, and gives less variations in press-formability in a coil, being manufactured in an industrially stable state.
To achieve the object, the present invention provides a method for manufacturing cold-rolled steel sheet comprising the steps of:
(a) providing a slab consisting essentially of 0.02% or less C, 0.5% or less Si, 2.5% or less Mn, 0.10% or less P, 0.05% or less S, 0.003% or less O, 0.003% or less N, 0.01 to 0.40% at least one element selected from the group consisting of Ti, Nb, V, and Zr, by weight, and balance being Fe;
(b) rough-rolling the slab by rough-rolling mill to form a sheet bar;
(c) finish-rolling the sheet bar by a continuous hot finish-rolling mill to form a hot-rolled steel strip,
xe2x80x83the finish-rolling comprising finish-rolling the sheet bar so that the material temperature at the final stand of the finish-rolling mill becomes Ar3 transformation point or more over the whole range of from the front end of the sheet bar to the rear end thereof;
(d) cooling the hot-rolled steel strip on a runout table and coiling the cooled hot-rolled steel strip,
xe2x80x83the cooling on the runout table beginning within a time range of from more than 0.1 second and less than 1.0 second after completed the finish-rolling,
xe2x80x83the cooling on the runout table being conducted at the average cooling speed in a temperature range of from the hot-rolling finish temperature to 700xc2x0 C. being 120xc2x0 C./sec or more,
xe2x80x83the average cooling speed in a temperature range of from 700xc2x0 C. to the coiling temperature being 50xc2x0 C./sec or less,
xe2x80x83the coiling temperature of the hot-rolled steel strip being less than 700xc2x0 C.; and
(e) applying pickling and cold rolling the hot-rolled steel strip, and final annealing to the cold-rolled steel strip.
It is another object of the present invention as the second aspect thereof to provide a method for manufacturing cold-rolled steel sheet having superior shape property, workability, and less-anisotropic property in a stable state.
To achieve the object, the present invention provides a method for manufacturing cold-rolled steel sheet comprising the steps of:
(a) heating a slab consisting essentially of 0.0003 to 0.004% C, 0.05% or less Si, 0.05 to 2.5% Mn, 0.003 to 0.1% P, 0.0003 to 0.02% S, 0.005 to 0.1% sol.Al, 0.0003 to 0.004% N, by weight, and balance of Fe;
(b) hot-rolling the slab to form a hot-rolled steel strip; and
(c) cold-rolling the hot-rolled steel strip to form a cold-rolled steel strip and annealing the cold-rolled steel strip,
xe2x80x83the step of hot-rolling comprising finish-rolling, cooling, and coiling,
xe2x80x83the finish-rolling having a total reduction in thickness of two passes before the final pass being in a range of from 25 to 45%, a reduction in thickness at the final pass being in a range of from 5 to 25%, and a finishing temperature being in a range of from the Ar3 transformation point to the (Ar3 transformation point+50xc2x0 C.), and
xe2x80x83the cooling being carried out by a rapid cooling at a cooling speed in a range of from 200 to 2,000xc2x0 C./sec within 1 second after completing the finish rolling, the temperature reduction from the finish temperature of the finish rolling in the rapid cooling being in a range of from 50 to 250xc2x0 C., and the temperature to stop the rapid cooling being in a range of from 650 to 850xc2x0 C., followed by applying slow cooling or air cooling at a rate of 100xc2x0 C./sec or less.
To achieve the object, the present invention further provides a method for manufacturing cold-rolled steel sheet comprising the steps of:
(a) heating a slab consisting essentially of 0.0003 to 0.004% C, 0.05% or less Si, 0.05 to 2.5% Mn, 0.003 to 0.1% P, 0.0003 to 0.02% S, 0.005 to 0.1% sol.Al, 0.0003 to 0.004% N, by weight, and balance of Fe;
(b) hot-rolling the heated slab to form a hot-rolled steel strip; and
(c) cold-rolling the hot-rolled steel strip to form a cold-rolled steel sheet and annealing the cold-rolled steel sheet;
xe2x80x83the step of hot-rolling comprising finish-rolling, cooling, and coiling,
xe2x80x83the total reduction in thickness of two passes before the final pass being in a range of from 45 to 70%, the reduction in thickness at the final pass being in a range of from 5 to 35%, and the finish temperature being in a range of from the Ar3 transformation point to the (Ar3 transformation point+50xc2x0 C.), and
xe2x80x83the cooling being carried out by a rapid cooling at a cooling speed of from 200 to 2,000xc2x0 C./sec within 1 second after completing the finish rolling, the temperature reduction from the finish temperature of the finish-rolling in the rapid cooling being in a range of from 50 to 250xc2x0 C., and the temperature to stop the rapid cooling being in a range of from 650 to 850xc2x0 C., followed by applying slow cooling or air cooling at a rate of 100xc2x0 C./sec or less.